Method of treating metal

ABSTRACT

A method of treating metal parts comprising the steps of (1) casehardening the exterior surface of the part by carburizing (2) phosphatizing the surface of the carburized part and (3) cold working the exterior surface of the part by shotpeening or other appropriate method.

United States Patent Inventor Gene P. Davis Muncie, Ind. Appl. No, 749,266 Filed Aug. 1, 1968 Patented Oct. 26, 1971 Assignee Borg-Warner Corporation Chicago, Ill.

METHOD OF TREATING METAL 5 Claims, No Drawings US. Cl 148/6.15, 148/ 165 Int. Cl C23f 7/10, B21d 7/06 Field of Search 148/6. 15 R,

Primary Examiner-Alfred L. Leavitt Assistant Examiner-Caleb Weston Att0meys-Donald W. Banner, Lyle S. Motley, C. G. Stallings and William S. McCurry ABSTRACT: A method of treating metal parts comprising the steps of l) casehardening the exterior surface of the part by carburizing (2) phosphatizing the surface of the carburized part and (3) cold working the exterior surface of the part by shotpeening or other appropriate method.

METHOD OF TREATING METAL SUMMARY OF INVENTION It is well known in the art of treating ferrous metal parts to dihydrogen phosphate including nitric acid as an accelerator in the amount of 2 percent and phosphoric acid in an amount to maintain the components in solution and the balance being water. The shotpeening and phosphating treatments of the shotpeen carburized parts to increase the fatigue life thereof, 5 samples in B were carried out as above the and it is further well known to phosphatize the shotpeened carburized parts to provide a surface which retains lubricant and otherwise assist in wear-in of the parts. The phosphatizing step comprises treating the metal surface with a phosphoric acid solution or a phosphoric acid-manganese carbonate solution, preferably by immersing the part in the solution. In the prior art it is known to manufacture metal parts by the steps of carburizing the surface of the part, mechanically working the surface and then phosphatizing the surface as, for example,

more particularly described and disclosed in U.S. Pat. No.

2,476,345 which outlines the steps of treatment in the above order.

It has been found that an improved method of treating carburized parts is to subject the parts to the phosphatizing process following the carburizing process and then, as a final step, to cold work the surface of the part by shotpeening. By this method it has been found that the life of the parts is materially increased. Phosphatizing following carburizing, removes portions of the surface and nearsurface oxides produced by the carburizing process and also removes certain undesirable stress areas left on the surface of the part. Cold working the part following the phosphatizing process improves the fatigue life of the part measurably as opposed to the known method of shotpeening prior to phosphatizing.

It has further been found that by utilizing the present improved method of having the shotpeening follow the phosphatizing step a portion of the coating applied in the phosphatizing process remains after the shotpeening so that the normal advantages of phosphatizing are still realized.

DESCRIPTION OF THE INVENTION In the manufacture of ferrous metal parts adapted to carry a load as for example gears and transmissions, it has been shotpeening was performed following the phosphatizing treatment in accordance with the method of the present invention. The gears were run with mating gears at a load of 260 pounds at 3,500 rpm.

TABLE I i Group Average life A 9.3 hours B 19.0 hours In accordance with the above, the parts were run until they had failed to such an extent that the noise was objectionable to the point of considering the gears unsuitable for use.

Summarizing the above results it can be seen that in the samples of Group A in which the known method of shotpeening followed by phosphate treatment was used, the average life of the parts was approximately 9.3 hours. In the samples of Group B the average life was 19 hours following the improved method. All of the parts used in the above test were from the same heat of steel, forged, were machined, carburized, and shotpeened at the same time.

The characteristics of parts manufactured in accordance with the improved method of the present invention are further 1 apart and a load applied at the center of the bar which caused a maximum of bending stress of 196,000 p.s.i. at a frequency of 10,000 cycles/minute. In the tests outlined in table II below, consisting of Groups A, B, C, there were 20 sample bars in TABLE II Average Average residual stress, hardness 101 p.s.i. Average residual Life cycles Tension Compression stress, Group Treatment Re X-ray side side p.s.i. B10 B carburized, shotpeened 62.0 152.7 117.6 123.4 --120,253 13,700 34,300 B-.. carburized,shotpeened hosphatlzed- 61.7 51.6 102.8 -108.8 -106,807 14,200 34,400 0 carburized, phosphatize ,shotpeened 62.1 52.8 -12l.6 -124,1 122,848 16,800 48,100

known to follow the steps of carburizing to provide a case As shown in table II, Group A parts were given a carburizing hardened surface adapted to have good wear resistance, to 50 and shotpeening treatment only. GroupB parts were given l) ing the useful life of a ferrous metal part comprising the steps of (1) carburizing (2) phosphatizing and (3) cold working (shotpeening for example).

The characteristics developed in parts manufactured by the improved method of the present invention may be shown, for

example in table I, which reflects the testing carried out on low speed gears for a three-speed transmission made from SAE 4027 steel. There were four samples in each group which was tested and each of the samples was carburized in a continuous carburizing gas furnace to develop a case depth of 0.035 inch 0.045 inch as measured at the pitch diameter of the gear tooth and oil quenched to provide a surface hardness of Rockwell 58 minimum after tempering at 350 to 375. In the four tests performed in Groups A and B the parts were shotpeened on a mechanical wheel-type shotpeening unit using SAE 230 cast steel shot each for the same length of time.

Following shotpeening the samples in Group A were then given a phosphate treatment to develop an iron-manganese phosphate finish producing a black fine crystalline adherent coating 0.0003 inch -0.0004 inch thick. The phosphate treatcarburizing (2) shotpeening and (3) phosphatized treatment in accordance with the known method of treating ferrous metal parts. The parts of Group C were manufactured by the improved method, namely (1) carburizing (2) phosphatizing and then (3) shotpeening. The above Table illustrates that the B life of the parts following the improved method as demonstrated by Group C was 16,800 cycles as compared to the Group B samples manufactured by prior art known methods giving 14,200 cycles. The B life is a term of art which refers to the number of cycles at which 10 percent of the parts have failed. The chart further illustrates that the B life of the parts treated by the present invention method was 45,100 cycles as compared to 34,400 cycles for the prior art method shown in Group B, the B life being the point at which half the parts in the test had failed.

Various features of the invention have been particularly ment utilized a solution containing 35 percent manganese 5 shown and described; however. it h uld b Obvious 10 One ,peening.

3. A method as claimed in claim 1 wherein said phosphatizing step comprises contacting the part by dipping same in phosphoric acid bath. 7

4. The method of claim 1 wherein the phosphatizing solution includes manganese dihydrogen phosphate, nitric acid and phosphoric acid.

5. A ferrous metal gear hat ling substantially increased life that has been treated by case hardening the exterior surface by carburizing, phosphatizing and surface stressing by cold working the exterior surface that order.

I i t I 

2. A method as claimed in claim 1 wherein said cold working step comprises mechanically working the surface by shotpeening.
 3. A method as claimed in claim 1 wherein said phosphatizing step comprises contacting the part by dipping same in phosphoric acid bath.
 4. The method of claim 1 wherein the phosphatizing solution includes manganese dihydrogen phosphate, nitric acid and phosphoric acid.
 5. A ferrous metal gear having substantially increased life that has been treated by case hardening the exterior surface by carburizing, phosphatizing and surface stressing by cold working the exterior surface in that order. 